ESA ESA Soil Science & Biogeochemistry 2 — Questions and Answers
Question 1: In the global carbon cycle, the largest active reservoir of carbon is found in:
- The atmosphere
- Living terrestrial biomass
- Soils (organic carbon) (Correct answer)
- The surface ocean
Correct answer: Soils (organic carbon)
Soils contain approximately 1,500–2,000 Pg C in organic form, making them the largest active terrestrial carbon reservoir—roughly twice the atmospheric pool.
Question 2: Denitrification is favored under which soil conditions?
- Aerobic, low-carbon soils
- Anaerobic, carbon-rich soils (Correct answer)
- Dry, high-pH soils
- Frozen soils with high bulk density
Correct answer: Anaerobic, carbon-rich soils
Denitrifying bacteria use NO3− as an electron acceptor when O2 is limiting, so anaerobic, carbon-rich (electron donor-rich) conditions maximize denitrification.
Question 3: Which soil texture class has the highest water-holding capacity per unit volume?
- Sand
- Silt loam (Correct answer)
- Gravel
- Coarse sandy loam
Correct answer: Silt loam
Silt loam soils have a fine pore structure that retains more plant-available water than coarser-textured soils while still allowing drainage.
Question 4: The Redfield ratio describes the stoichiometric relationship in marine phytoplankton as approximately:
- C:N:P = 100:10:1
- C:N:P = 106:16:1 (Correct answer)
- C:N:P = 50:8:1
- C:N:P = 200:20:2
Correct answer: C:N:P = 106:16:1
The Redfield ratio (C:N:P ≈ 106:16:1) reflects the consistent elemental composition of marine phytoplankton and guides understanding of nutrient limitation.
Question 5: Phosphorus availability in soils is typically lowest at:
- Neutral pH (6.5–7.0)
- Acidic pH below 5.5 and alkaline pH above 7.5 (Correct answer)
- Slightly acidic pH of 6.0
- Neutral to slightly alkaline pH
Correct answer: Acidic pH below 5.5 and alkaline pH above 7.5
At low pH, phosphorus is fixed by iron and aluminum; at high pH, it precipitates with calcium—so availability peaks near neutral pH.
Question 6: Which process is responsible for the conversion of organic nitrogen in decomposing litter to inorganic ammonium?
- Nitrification
- Mineralization (ammonification) (Correct answer)
- Denitrification
- Volatilization
Correct answer: Mineralization (ammonification)
Mineralization (ammonification) is the microbial decomposition of organic nitrogen-containing compounds that releases NH4+ into the soil solution.
ESA ESA Soil Science & Biogeochemistry 2
In the global carbon cycle, the largest active reservoir of carbon is found in: